1. Technical Field
The present invention relates to a piezoelectric device having a resonator element housed in a package, a cellular phone system using the piezoelectric device, and electronic equipment using the piezoelectric device.
2. Background Art
Piezoelectric devices, such as piezoelectric transducers or piezoelectric oscillators, are in widespread use in hard disk drives (HDDs), mobile computers, small computer equipment including IC cards, and mobile communication equipment including cellular phones, automobile telephones, and paging systems.
A known piezoelectric device has, for example, a resonator element shown in FIG. 11 housed in a package (refer to Japanese Unexamined Patent Application Publication No. 2002-261575).
Referring to FIG. 11, a resonator element 1 formed by etching, for example, crystal is a so-called tuning-fork-type resonator element, which has a base 10 and a pair of resonating arms 2 and 3 that extend in parallel to each other from the base 10.
Grooves 2a and 3a extending in the longitudinal direction of the resonating arms 2 and 3 are formed in the resonating arms 2 and 3, respectively. Drive electrodes (not shown) are formed in the grooves 2a and 3a. Cutouts 4, 4 are formed at opposing ends in the width direction at the outer end of the base.
The resonator element 1 is housed in, for example, a box-shaped package (not shown) and is adhered on the inner surface of the package where electrode parts connected to external terminals are provided. Conductive adhesive is applied on the electrodes. The base 10 of the resonator element 1 is mounted on the conductive adhesive and then cured. Extraction electrodes 2b and 3b of the base 10 are adhered to the electrode parts, separated from each other at the package side, with the corresponding conductive adhesive to be electrically and mechanically connected and fixed to the electrode parts.
The base 10 of the resonator element 1 forms an adhesion area to which the package is adhered, as described above, and has a certain size so as to make the transmission of a vibration of the resonating arms 2 and 3 toward the package difficult. The resonator element 1 has the cutouts 4, 4 formed in the base 10 in order to make the transmission of a vibration of the resonating arms 2 and 3 difficult.
A resonator element 5 shown in FIG. 12 is also known (refer to Japanese Unexamined Patent Application Publication No. 53-23588).
The resonator element 5 has a rectangular frame 6. A tuning-fork-type resonator element body is integrally formed inside the frame 6 through a connecting part 7a. Specifically, the resonator element body has a base 7 and resonating arms 8 and 9 that extend in parallel to each other from the base 7 inside the frame 6. The base 7 and the resonating arms 8 and 9 are connected to the frame 6 through the integrally formed narrow connecting part 7a. 
However, since the resonator element 1 described with reference to FIG. 11 has a structure in which the base 10 is adhered on the inner surface of the package, the resonator element 1 to be housed in the package is reduced in size when the piezoelectric device is downsized. Particularly, it is difficult to use the conductive adhesives to adhere the extraction electrodes to the base 10 with the extraction electrodes being electrically separated from the base 10. In other words, it becomes difficult to apply a small amount of the conductive adhesive to the electrodes separated from each other at minute intervals on the inner surface of the package so as not to short-circuit the electrodes. Since it is necessary to limit the amount of applied conductive adhesive for preventing the electrodes from being short-circuited, the adhesion strength of the resonator element 1 becomes insufficient.
In contrast, since the resonator element 5 shown in FIG. 12 uses the frame 6 as part of the package and the resonator element 5 is sandwiched between an insulating substrate (not shown) and a lid (not shown) and fixed, there is no need to employ an adhesion structure in the package of the resonator element. Hence, the problems described above when the resonator element 1 in FIG. 11 is used can be solved and the resonator element 5 in FIG. 12 is advantageous in terms of size reduction of the piezoelectric device.
However, in the resonator element 5 in FIG. 12, the fixed frame 6 used as part of the package is connected to the resonator element body that flexes and vibrates in response to a piezoelectric action through the extremely narrow connecting part 7a in order not to inhibit the resonator element body from vibrating as much as possible. As a result, when an external impact is applied, the stress is concentrated on the extremely narrow connecting part 7a and, therefore, the connecting part 7a is in danger of being damaged. Accordingly, it is disadvantageously difficult to form the piezoelectric device having a sufficient impact resistance.
When the base 7 having a width larger than that of the connecting part 7a is directly formed inside the frame 6, without forming the connecting part 7a, in the resonator element 5 shown in FIG. 12 in order to avoid breakage, the base 7 inhibits the vibration of the resonator element body unless the base 7 has a sufficient length and size. Furthermore, a drop test conducted by the inventors et al. shows that, when the piezoelectric device is formed by using the resonator element having the structure in FIG. 12, an external impact is transmitted to the resonator element body through the frame 6 used as part of the package and the base 7, and the piezoelectric device is affected as shown in FIGS. 13(a) and 13(b).
FIG. 13(a) is a graph showing the increase in crystal impedance (CI) values of the piezoelectric device due to a dropping impact. FIG. 13(b) is a graph showing the variation in frequency of the piezoelectric device due to the dropping impact. Reference letters X, Y, and Z in FIGS. 13(a) and 13(b) denote directions represented by reference letters X, Y, and Z in FIG. 1 and indicate the directions of dropping in the drop test. As shown in FIGS. 13(a) and 13(b), the piezoelectric device is adversely and significantly affected in its performance.
In order to solve the problems described above, it is an object of the present invention to provide a piezoelectric device capable of achieving a sufficient impact resistance even when the product is reduced in size, a cellular phone system using the piezoelectric device, and electronic equipment using the piezoelectric device.